The present invention relates to a high density printed wiring board for mounting high density semiconductor devices, and more particularly the present invention relates to a high density printed wiring board having pads and surface wiring patterns which can be easily modified on the surface of the printed wiring board when the circuitry, of the semiconductor devices is modified.
Mechanical parts and electronic parts to be mounted on a printed wiring board have in the past been fixed mechanically using through holes provided in the printed wiring board as shown in FIG. 1. FIG. 1 illustrates an electronic part 101 which is fixed to a printed wiring board 201 using through holes 103. In FIG. 1, the electronic part 101 is fixed to the printed wiring board 201 by soldering its leads 102 to the through holes 103 as shown using solder 105. Although not depicted in FIG. 1, such through holes 103 are connected to internal wires provided in the printed wiring board 201 so that an electronic part 101 is electrically connected to such internal wires by the through holes 103.
However, recently electronic parts having very high packing densities, i.e. Large Scale Integration (LSI) semiconductor devices have been developed. Thus, printed wiring boards mounting such devices must have high density internal wirings. Large size through holes would interfere with the high packing density of a printed wiring board and as a result, almost all through holes have now been replaced with via holes as illustrated in FIG. 2. In FIG. 2, a high density semiconductor device 111 uses bump terminals (hereinafter referred to as BUMPs) as input or output terminals and the BUMPs 112 are respectively fixed to pads 107 mounted on the via holes with solder. Comparing FIGS. 1 and 2, it can be seen that the via hole construction is superior to the through hole construction in realizing high packing densities in printed wiring boards.
Semiconductor devices packed in flat lead packages are also widely used in FIG. 3, the reference numeral 122 designates a flat lead of a flat lead package device 121. However, in the case of high density semiconductor devices, it must be considered that the transmission velocities of input and output signals are lowered as a function of the length of the lead. Therefore, recently semiconductor devices have been packed in packages having BUMPs, and furthermore BUMPs have been applied directly to unpackaged semiconductor elements, semiconductor devices packed in a package are referred to as semiconductor package devices and unpackaged semiconductor devices are referred to as semiconductor element devices. The present invention relates to both the semiconductor package devices and the semiconductor element devices.
Through holes and the via holes are substantially equal, however the holes having a small size and a pad at the surface of the printed wiring board are called via holes in this disclosure.
FIG. 4 is a schematic diagram illustrating a prior art semiconductor element device. FIG. 4(a) is a schematic side view of a semiconductor element device 1 mounted on a printed wiring board 2, while FIG. 4(b) is a schematic plan view thereof. In FIGS. 4(a) and 4(b), the reference numeral 11 represents BUMPs which are soldered onto terminal pads 3 provided on the surface of a printed wiring board 2 and connected with respective inner circuits of the printed wiring board 2 which are not depicted in FIG. 4(a). Thereby, the semiconductor element device 1 is electrically connected to the inner circuits and mechanically fixed to the printed wiring board 2 keeping the distance d between the upper surface of the printed wiring board 2 and the lower surface of the semiconductor device 1. The distance d is generally 0.2 through 0.3 mm. As shown in FIG.4(b), the BUMPs 11 are usually arranged in a grid pattern.
In case where the semiconductor device is a semiconductor package device, the size of the device body is larger than if it were a semiconductor element device, but the arrangement of BUMPs and the mounting of the device on the printed wiring board are the same as shown in FIGS. 4(a) and 4(b).
Thus, BUMPs have been useful for mounting high density semiconductor devices on the prior art high density printed wiring boards. However, problems are presented prior art high density printed wiring boards when the number of BUMPs increases.
Generally, the internal wiring associated with the terminal pad is arranged for connection to designated circuits of the printed wiring board, and it is not easy to change the internal wiring every time the arrangement needs to be modified, because the costs for redesigning and manufacturing modified internal wiring is high.
Therefore, usually, pads called modification pads are provided on the surface of the printed wiring board for modifying the destinations of the terminal pads. The modification pads are connected with the terminal pads by internal wiring provided in the printed wiring board and placed near the via pads. The via pads are generally connected to their respective designated circuits through internal wiring and are usually connected to the modification pads by a surface wiring pattern provided on the surface of the printed wiring board. FIG. 5 illustrates the prior art modification pads, wiring and wiring patterns provided for prior art high density printed wiring boards. FIG.5(a) is a top view illustrating terminal pads 3, prior art modification pads 5 and 55, prior art via pads 4 and 44, and the wiring and wiring patterns associated with such. FIG. 5(b) is a partial cross-sectional view taken at the line p--p' of FIG. 5(a). In FIGS. 5(a) and 5(b), the reference numeral correspond with the reference numerals of FIG. 4 and are used to designate the same parts as in FIG. 4. In FIG. 5(a), the dotted square R1 indicates a terminal pads region in which the terminal pads are gathered. The reference numerals 3 refer to the terminal pads generally and particular terminal pads arranged on the line p--p' are indicated by reference numerals 31 through 36 from right to left respectively for explaining the function of the prior art modification pads.
In FIGS. 5(a) and 5(b), each of the pads 31 through 36 has a respective modification pad and via pad. For example, terminal pads 31 and 32 have modification pads 5 and 55 and via pads 4 and 44 respectively. The via pads 4 and 44 are connected to respective previously designated circuits 991 and 992 through respective internal wiring and are connected respectively to terminal pads 31 and 32 through the modification pads 5 and 55. That is, terminal pad 31 is connected with modification pad 5 by surface wiring pattern 72 and modification pad 5 is connected with via pad 4 by surface wiring pattern 71, while terminal pad 32 is connected with modification pad 55 by internal wiring 722 and modification pad 55 is connected with via pad 44 by surface wiring pattern 711. "Surface wiring pattern" 72 is used for the wiring between terminal pad 31 and modification pad 5 only because terminal 31 is the most outside terminal pad. On the other hand, the inside terminal pads 32 through 35 are connected to their respective modification pad using internal wiring. However, in FIGS. 5(a ) and 5(b), it should be noted that all between the modification pads and the via pads is comprised of respective surface wiring patterns such as the surface wiring pattern 711. Thus, each modification pad usually functions to relay electrical connection between the corresponding terminal pads and via pads. In other words, the modification pads and the surface wiring patterns from the modification pads to the via pads usually function only to extend the wiring from the terminal pads to the via pads. However, if it is required to modify the destination of a particular terminal pad, the modification can be easily made by cutting the surface wiring pattern between the corresponding modification pad and via pad and providing new wiring running along the surface of the printed wiring board. FIG. 6 is a fragmentary top view of the high density printed wiring board 1 of FIG. 5 illustrating an example of cutting the surface wiring patterns 71 and 711. In FIG. 6, the reference numerals are the same as those of FIG. 5 and indicate like parts. When terminal pads 31 and 32 are to be modified so as to be connected with other destinations referred to as circuits 997 and 998 respectively, the modification can be performed by cutting surface wiring patterns 71 and 711 and providing new surface wiring leading respectively to the circuits 997 and 998 along the surface of the printed wiring board 1.
Thus, in the prior art, because the modification pads and the via pads were positioned outside the terminal pads region R1 with the associated wiring and wiring patterns, destination modification was easily performed without redesign and/or need for new printed wiring boards. However, in recent years, the number of input and output terminals of high density semiconductor devices, namely the terminal pads on high density printed wiring boards, has increased to as many as 200 or more, and this has created the following problems in the use of the high density printed wiring boards of the prior art:
(1) the mounting density of high density semiconductor devices on the high density printed wiring board is reduced because the region R2, shown in FIG. 5(a), for prior art modification pads and via pads, needs to be enlarged because of the increase in the number of increase of the terminal pads;
(2) the high density printed wiring board must have two, three or more layers because the amount of internal wiring for connecting the terminal pads to the modification pads is increased, and this results in increased cost; and
(3) the signal transmission velocity of the signals running in the high density printed wiring board is decreased because the internal wiring connecting the terminal pads with the modification pads is necessarily longer.
In FIGS. 4 and 5, the BUMPs 11 of the high density semiconductor device 1 are bonded on the terminal pads 3 of the high density printed wiring board 2 so as to keep the distance "d" of about 0.2 mm-0.3 mm between the device 1 and the board 2 as described above. However, in the case of a high density semiconductor device comprising as many as 200 BUMPs, the distance d must be reduced to a small value like 0.1 mm to facilitate the bonding process. In cases where the distance "d" is so small, and when the device is cooled by boiling a coolant, boiling bubbles maybe remain within the structure and lower the cooling efficiency. Moreover, when the number of BUMPs is increased, the cumulative position error by pitch p between the BUMPs also increases. In addition, stress is applied to the bonding parts of the BUMPs and terminal pads, due to the difference of thermal expansion coefficient of the high density semiconductor device and the high density printed wiring board, and as a result, joint reliability between the semiconductor device and the printed wiring board is decreased. To solve such problems, a contact wire has been inserted between each BUMP and in corresponding terminal pad.
FIG. 7 is a schematic side view of a high density semiconductor device mounted on a high density printed wiring board through such contact wires. In FIG. 7, the reference numerals correspond with those of FIG. 4 and indicate the same parts as those of FIG. 4, and the reference numeral 113 indicates the contact wires. Length "l" of the contact wires 113 is determined considering the number of BUMPs and should be adequate for setting the distance d at 0.5 mm or more. Contact wires are effective also in connection with the present invention when the number of BUMPs is large as will be disclosed hereinbelow.